1. Field of the Invention
The present invention relates to an electric power steering apparatus.
2. Description of the Related Art
In an electric power steering apparatus, a pinion shaft coupled to a steering shaft via a torsion bar is rotatably mounted in a gear housing. A rack shaft engaging with the pinion shaft is supported in the gear housing linearly movable. A worm gear coupled to a drive shaft of an electric motor is rotatably mounted to the gear housing. A worm wheel fixed to an intermediate portion of the pinion shaft and engaging with the worm gear is rotatably mounted to the gear housing. The electric motor is dimensioned to apply a steering assist torque corresponding to a steering torque applied by a driver to the steering shaft to the rack shaft via an engagement between the worm gear and the worm wheel, and an engagement between the pinion shaft and the rack shaft.
In the electric power steering apparatus mentioned above, it is necessary to easily set a center distance between the worm gear and the worm wheel without being affected by a dimensional error of a part such as the worm gear or the like, at a time of assembling, and easily regulate the center distance at a time when the engagement between the worm gear and the worm wheel deteriorates with age after the assembly, thereby removing a backlash.
Further, in the electric power steering apparatus mentioned above, it is demanded to easily set the center distance between the worm gear and the worm wheel without being affected by the dimensional error of the parts such as the worm gear or the like at a time of assembling, and generate no abnormal noise at a time of the engagement between the worm gear and the worm wheel, after the assembly.
In an electric power steering apparatus described in Japanese Unexamined Patent Publication (JP-A) No. 2001-10512 (patent document 1), a ring body and a bearing case are interposed between an outer periphery of a bearing supporting a leading end shaft of a worm gear and a gear housing, and an inner periphery of a bearing case and an outer periphery of the ring body are brought into contact with each other via an inclined surface. Further, a disc spring and an adjusting screw are provided in an outer side in an axial direction with respect to the worm gear, and the disc spring pressed by the adjusting spring is brought into contact with one end surface of an outer ring of a bearing that is integrally formed with the ring body. The worm gear is biased toward the worm wheel so as to regulate the center distance therebetween by pressing the bearing and the ring body in the axial direction by a spring force of the disc spring and moving the bearing along the inclined surface of the bearing case, at a time of assembling the electric power steering apparatus or after the assembly, and thus the backlash thereof is eliminated.
The following problems exist in the electric power steering apparatus described in the patent document 1.
(1) As the backlash regulating means, the ring body and the bearing case are provided in the outer peripheral side of the bearing of the worm gear, and the disc spring and the adjusting screw are provided in the outer side in the axial direction of the bearing of the worm gear. Accordingly, the outer sides are enlarged in both directions including a diametrical direction and an axial direction around the bearing of the worm gear, and the electric power steering apparatus is enlarged in size.
(2) Since the disc spring is provided in the outer side in the axial direction of the bearing of the worm gear, an installing space of the disc spring becomes small in the case that it is intended to suppress an enlargement in size of the electric power steering apparatus, and a flexibility for designing the disc spring is reduced.
(3) Since the adjusting screw pressurizing the disc spring engages with the gear housing, and includes the screw portion, the electric power steering apparatus cannot be easily assembled.
Further, the following problems exist in the electric power steering apparatus described in the patent document 1.
(4) As the backlash regulating means, the ring body and the bearing case are provided in the outer peripheral side of the bearing of the worm gear, and the disc spring and the adjusting screw are provided in the outer side in the axial direction of the bearing of the worm gear. Accordingly, even if it is possible to eliminate the abnormal noise at a time when both the elements are engaged, by eliminating the backlash between the worm gear and the worm wheel, an engaging friction (a frictional resistance caused by the engagement) between both the elements becomes large.
(5) An outer size around the bearing of the worm gear becomes large in both the diametrical direction and the axial direction, and the electric power steering apparatus becomes large in size.